Dynamics of oocyte nucleoli in mammals

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During the growth stage, oocyte nucleoli are intensively engaged in ribosomal RNA (rRNA) synthesis. As the oocytes approach the finalsize, synthetic activity of RNAs decreases, and coincidently nucleoli change the morphology and reduce the volume. Nucleoliof growing oocytes werecompacted and reduced the sizeby transcription inhibitors (actinomycin D or α-amanitin). On the other hand, the nucleolus volume of oocytes significantly increased by thetreatment with proteasome inhibitors, MG-132 or lactacystin. During oocyte maturation, the nucleolus was disassembled into the cytoplasm. When the oocytes were arrested at the germinal vesicle stage, and the nucleolus was aspirated from the nucleus and transferred to the cytoplasm, the nucleolus disappeared and later it was reassembled in the nucleus.